The present invention relates to lubrication systems for turbine engines and for associated equipment, and more particularly, to air and lubricant heat exchangers for use in maintaining desired temperatures of the lubricants in such engines and equipment.
Lubrication systems for turbine engines, such as a turbofan engine, and for associated equipment, such as an integrated drive generator, provide pressurized lubricant, an oil, to lubricate, cool and clean the engine main bearings, gear box gears, and the like, and again for the lubrication of bearings and other parts in equipment associated with such turbine engines. During such lubrications, heating of the lubricant is caused to occur due to mechanical energy losses in the lubricated apparatus. Thermal management of such lubricants is very important for continued successful operation of such lubrication systems in the apparatus lubricated thereby.
The amount of heat necessary to be ejected from lubricants in such systems is increasing because of the use of larger electrical generators, for instance, in aircraft turbine engines due to increasing consumption of electrical power in the aircraft powered thereby, and because of the advances in aircraft turbine engines such as the use of geared turbofans for such aircraft with large fan-drive gearboxes. Despite the added heat generated by such modified and expanded equipment, the necessary lubricating oil operating temperature ranges to provide satisfactory lubricating performance have not changed for the most part and, in some instances, the upper operating temperature limits have been reduced.
The lubrication system for a turbofan engine in an aircraft typically has a first heat exchanger having passageways through which lubricating oil passes to be cooled by the fuel stream flowing past and around these passageways. This arrangement permits the lubricating oil to reject heat therein to the fuel in the aircraft thereby heating that fuel to help recover some of the energy lost in the combustor of the engine. Because in some flight situations more heat is generated in the lubricating oil than can be accepted for warming the fuel, a portion of the lubricating oil can be forced to bypass the heat exchanger for the fuel and the lubricating oil, and the oil can be directed to a further heat exchanger where the heat therein is transferred to the air in the secondary airstream provided by the fan of the turbofan engine. In a typical arrangement, a duct is provided in the fan cowling through which a portion of the airstream is diverted, and the air and lubricating oil heat exchanger is placed in this duct so that the lubricating oil passing through passageways in that heat exchanger is cooled by the duct airstream flowing past these passageways in the exchanger. If such additional cooling of the oil is not needed in a flight situation, the lubricating oil can again be forced to bypass this air and lubricating oil heat exchanger.
However, the fan airstream that is diverted to pass through the lubricating oil and air heat exchanger in such duct systems flows at least in part through that exchanger, and the exchanger should be large enough, insofar as assuring that a sufficient part of the cooling engine fan airstream flows over a sufficient amount of lubricating oil flowing in passageways therein, to provide adequate oil cooling for the most extreme flight conditions encountered. Larger heat exchangers require larger cross sectional area ducts, and such an air and lubricating oil heat exchanger duct based system continually leads to thrust losses in the turbofan engine. Hence, there is a strong desire for an air and lubricating oil heat exchanger duct based system that reduces such thrust losses and also reduces the volume required therefor in the more compact spaces in advanced turbofan engines.